When you place a bet on a horse race, instead of taking the fixed odds price on offer you might be able to place a bet at an unspecified Starting Price. Sometimes for future races, which haven’t been priced up by your selected bookmaker, you can only place a bet at the Starting Price (SP).
When you place a bet at the SP, you do not know in advance what odds you are getting! So, why on earth would you want to take this option? Odds on horses can fluctuate, in the build up to the race. When the race start time comes closer, it is clearer what the conditions will be like (e.g. is the ground heavy or soft, will it be raining or really hot at the time of the race), if any other horses have withdrawn, and if the horse has any issues etc. Odds can thus fluctuate for many reasons, including the amount being bet on a horse. So, with lots of money coming in, the price is likely to go down and vice-versa. In the UK, a panel decides the starting price (it should be the same regardless of which bookmaker you use), based on how they view the fluctuations of prices.
If you place a bet by taking the Starting Price, and your horse doesn’t start, your stake will be returned. This will not usually be the case if you took a fixed price, unless the bookmaker was specifically running a non-runner no-bet promotion (NRNB).
Some bookmakers run a Best Odds Guaranteed (BOG) promotion (please check all T&Cs) on certain horse races. Usually, if you take a fixed odds price after a certain time, but the starting price is higher, you will get paid according to the Starting Price.